Small bowel neuroendocrine tumors (SBNETs), also known as small bowel carcinoid tumors, are the most common gastrointestinal neuroendocrine tumors and most frequently involve the terminal ileum.
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Epidemiology
SBNETs account for ~40% of gastrointestinal neuroendocrine tumors 1.
Clinical presentation
Small bowel neuroendocrine tumors are slow growing and may present with vague symptoms 1,3:
weight loss
fatigue
diarrhea
abdominal pain
More specific symptoms include:
obstructive jaundice: in duodenal carcinoid
Pathology
Carcinoid tumors are neuroendocrine tumors arising from APUD cells. They can cause a desmoplastic reaction in nearby tissue, leading to fibrosis and tethering of the adjacent bowel 1,3 (for further discussion, see the main article).
The primary tumor in small bowel carcinoid is typically only up to 3.5 cm in size. Metastases, commonly to the mesentery, liver, and lymph nodes, often exceed the size of the primary neoplasm. Multiple primaries and metachronous tumors in other organs can often occur 3.
About 45% (range 30-60%) of patients with small bowel carcinoid have metastatic disease at presentation 1,3. The larger the primary, the greater the likelihood of metastases 4.
Radiographic features
CT
Primary small bowel neuroendocrine tumors are not always seen on imaging given their small size 4,6. Targeted protocols including a late arterial phase and CT enterography improve the sensitivity for detecting small bowel tumors 6. Characteristics of the primary lesions on CT include:
polypoid or plaque-like appearance 3,6
hyper-enhancing on arterial phase 3,6
can cause distortion and focal fixation of the affected small bowel loop - “hairpin” kinks in the course of the small bowel 1
calcifications are present in up to 70% of cases 4
Mesenteric metastases can appear well-defined or spiculated on CT, with stranding due to fibrosis and desmoplastic reaction (leading to a characteristic “spoke-like” appearance of mesenteric vessels) 1,3.
In the liver, metastases strongly enhance in the arterial phase due to their vascularity 4, then become isodense to liver parenchyma in the delayed phase.
Treatment and prognosis
Surgical resection is considered the main treatment for localized tumors 3. Carcinoid syndrome mainly treated by somatostatin analog therapy. Everolimus is used as 2nd line treatment 5. Surgical resection, embolization and ablation can be used to treat hepatic metastases 3.
Practical points
about 50% half of patients with small primary tumors will present with mesenteric disease 6